You are welcomed to check out [url=http://www.phy.ntnu.edu.tw/ntnujava/index.php?topic=539.0]Electric field structure and Field lines for two point charges +/-[/url]

e.g. (x,y) is the coordinate for the trajectory of field.
Set up (x,y) at the starting point for the field line. And You need to know/calculate electric field Ex,Ey at any (x,y)
Solve dx/dt=Ex and dy/dt=Ey ,then x,y will move alone the field line.

The above simulation use the same method to draw the field lines.